This invention relates to quantitative calorimetry for high resolution thermal analysis using probe based technology. For the purposes of clarity of this application, the reduction to practice has been explained using a Scanning Probe Microscope (SPM) but the technique described herein can be practiced with any device that can provide precise control of X, Y and Z motion of the probe, such as, by way of example, a micromanipulator.
The family of thermal analysis techniques, collectively called micro-thermal analysis (micro-TA), has been in existence for nearly a decade now. Micro-TA methods are based on a scanning probe microscope in which the conventional passive probe, typically a cantilever arm with integral tip, is enhanced, typically by adding the capabilities to measure temperature and be resistively heated. This type of SPM is called scanning thermal microscopy SThM, and specifically in this application, SThM in which the probe is actively heated rather than the sample. This form of microscopy allows thermal properties such as thermal conductivity and diffusivity to be mapped on a sub-micron scale. The heated probe will cause highly localized surface effects due to temperature. Used with an SPM, which is extremely sensitive to height variations, measured by changes in the deflection of the cantilever probe, heating the surface will cause cantilever deflections due to local thermal expansion, material softening, or both. Additionally, the amount of power fed to the heater and the resistance of the heater can be plotted independently or compared to the changes in cantilever deflection. Also, the power fed to two probes, one on the sample surface and one away from the sample surface can be compared to create a differential signal. The differential signal is used either to produce localized analysis plots versus temperature that provide temperature dependent information at a specific position on the sample, or to construct an image whose contrasts represent variations in thermal conductivity and/or diffusivity across a scanned area.
Measurements as described above have been accomplished to map temperature dependent material properties on a scale smaller than achieved by conventional bulk thermal analysis techniques. However, to date scanning thermal microscopy has been used only to make qualitative measurements in terms of enthalpies associated with transitions. The SThM technique as practiced currently does not allow quantitative measurements of the enthalpies. The lack of quantitative information is due to the fact that the tip interaction with the surface from a heating standpoint has several inherently undefined parameters. In particular, it is difficult to know how much of the sample's volume is heated, how uniformly is the heated volume affected, and what is the area available for heat flow from the tip to surface. Also, because of the force between the tip and sample as the material undergoes phase transitions the contact area can change. All of these factors contribute to the fact that heat flow from the tip into the bulk of the sample is not well-defined.
SThM has proven useful, for instance, to, on a sub-micron scale, detect areas of different material in a material blend and determine the transition temperatures of the different materials. However since SThM does not measure enthalpy it does not on any scale provide the type of data available on bulk analysis systems such as the Differential Scanning Calorimetry (DSC) system. In a DSC system, the temperature can be calibrated by running melting standards but also the enthalpy can be calibrated. This can be done by measuring samples with known heats of fusion or known heat capacities. Typically calibrating the heat capacity is done using Sapphire samples, but in any case the calibration requires knowing the mass of the material heated, which in a bulk analysis system is always the case since a known (relatively large) quantity of material is subjected to a uniform heating. Again because of the lack of knowledge of the quantity of material affected by the SThM it has not been possible to date to quantitatively determine the enthalpy absorbed by the sample or eg the enthalpy of fusion. Following are a few examples of applications in which it is beneficial to have quantitative measurements of the enthalpy as demonstrated by using traditional DSC techniques. Probably the commonest use of the DSC curve is in “fingerprinting”, in which simple or complex materials can be compared for identification, or quality control purposes, using measurements of thermal transition peak positions, sizes, or shapes as appropriate. The temperature at which peaks occur can lead to an identification of a particular component, and the size (usually the area, though the height is sometimes used) can give a measure of the amount of that component. Examples include the determination of quartz in clays, which is difficult by other methods, and the analysis of polymer blends. Analysis of the form of the fusion peak of a fairly pure (>98%) substance can, with certain restrictions, lead to a determination of its purity. This approach is used routinely with pharmaceuticals and fine chemicals in general. All of the above techniques would be of great utility if available on the resolution scale possible with SThM, such as for characterization and analysis of polymer or biological materials on the molecular scale. Therefore it is the object of this invention to provide quantitative thermal analysis techniques applicable to SThM.